CO2 Is Not A Dangerous Gas

Perhaps one of The Courant's science writers, Professor Robert Thorson for example, can explain the logic here:

The carbon atoms in all food on the planet for all plants and animals comes originally from atmospheric CO2. We go out of our way to protect our food from oxygen gas, a metabolic poison chemically similar to chlorine gas (the only way animals can survive on an oxygen atmosphere is by ingesting antioxidants), because over time oxygen destroys organic compounds (that's where the term oxidation came from).

Yet, we define CO2 as the pollutant in the atmosphere. How did humans ever have the sense to come down from the trees?

James R. Barrante, CheshireThe writer is an emeritus professor of physical chemistry at Southern Connecticut State University.